How to clean the content of a DataGrid?

I have a button in a window (WPF) and I code C# that contains the following:

<Button x:Name="btnBuscar" Content="Buscar por" HorizontalAlignment="Left" Margin="157,10,0,0" VerticalAlignment="Top" Width="85" Height="30" Click="btnBuscar_Click"/>

a comboBox:

<ComboBox x:Name="cmbBuscarPor" HorizontalAlignment="Left" Margin="247,10,0,0" VerticalAlignment="Top" Width="120" Height="30">
        <ComboBoxItem Content="Nombre"/>
        <ComboBoxItem Content="Apellido paterno"/>
        <ComboBoxItem Content="Puesto"/>
    </ComboBox>

a textbox:

<TextBox x:Name="tbBusqueda" Height="30" Margin="0,10,160,0" TextWrapping="Wrap" Text="Ingrese texto" VerticalAlignment="Top" VerticalContentAlignment="Center" HorizontalAlignment="Right" Width="230" GotFocus="tbBusqueda_GotFocus"/>

a DataGrid something similar to the following (more columns):

<DataGrid x:Name="dataGridUsuarios" CanUserAddRows="True" HorizontalAlignment="Left" Height="230" Margin="10,110,0,0" VerticalAlignment="Top" Width="742" KeyUp="dataGridUsuarios_KeyUp" MouseLeftButtonUp="dataGridUsuarios_MouseLeftButtonUp">
        <DataGrid.Columns>
            <DataGridTextColumn Binding="{Binding Path=idUsuario}" ClipboardContentBinding="{x:Null}"  Header="idUsuario" HeaderStringFormat="idUsuario" Visibility="Hidden"/>
            <DataGridTextColumn Binding="{Binding Path=Nombre}" ClipboardContentBinding="{x:Null}" Header="Nombre"  HeaderStringFormat="Nombre" Width="105"/>
            <DataGridTextColumn Binding="{Binding Path=ApellidoP}" ClipboardContentBinding="{x:Null}" Header="Apellido Paterno" HeaderStringFormat="ApellidoP" Width="105"/>
            <DataGridTextColumn Binding="{Binding Path=ApellidoM}" ClipboardContentBinding="{x:Null}" Header="Apellido materno" HeaderStringFormat="ApellidoM" Width="105"/>
            <DataGridTextColumn Binding="{Binding Path=Sexo}" ClipboardContentBinding="{x:Null}" Header="Sexo" HeaderStringFormat="Sexo" Width="105"/>
        </DataGrid.Columns>

The DataGrid is filled at startup with the records of a database with the following method

public void llenadoDataGrid()
    {
        String consulta = "select idUsuario,Nombre,ApellidoP ,ApellidoM ,Sexo,Telefono,Edad,Puesto,NombreUsuario,Contraseña from usuarios;";
        SqlDataAdapter dataAdapter = new SqlDataAdapter(consulta, new BaseDeDatos().obtenerConexion());
        DataSet ds = new DataSet();
        dataAdapter.Fill(ds);
        DataTableCollection collection = ds.Tables;
        DataTable table = collection[0];

        foreach (DataRow row in table.Rows)
        {
            var data = new PruebaDeLLenadoDataGrid {idUsuario = row["idUsuario"].ToString(), Nombre = row["Nombre"].ToString(),
                ApellidoP = row["ApellidoP"].ToString(), ApellidoM = row["ApellidoM"].ToString(),
                Sexo = row["Sexo"].ToString()
            };
            dataGridUsuarios.Items.Add(data);
        }
    }

I use the following class that I was advised to be able to fill the data grid with the previous method

class PruebaDeLLenadoDataGrid
{
    public String idUsuario { get; set; }
    public String Nombre { get; set; }
    public String ApellidoP { get; set; }
    public String ApellidoM { get; set; }
    public String Sexo { get; set; }
}

When I hit the search button, I do the following

private void btnBuscar_Click(object sender, RoutedEventArgs e)
    {
        String dato = tbBusqueda.Text;
        int index = cmbBuscarPor.SelectedIndex;
        if (dato != ""  && dato != "Ingrese texto" && index != -1)
        {
            dataGridBuscador(index, dato);
        }
        else
        {
            if (index == -1)
            {
                MessageBox.Show("seleccione una opcion de busqueda");
            }
            MessageBox.Show("Ingrese un texto para la busqueda");
        }
        MessageBox.Show("busqueda valor index "+index);
    }

the search is carried out with the dataGrid Buscador method in which I send the index to use each of the different searches and the problem lies here since when executing this it performs the search and shows it in the dataGridUsuarios it puts me a new row followed by the above instead of just displaying the search row(s)

        private void llenadoConBusqueda(int opcion,String dato)
    {
        String prueba = "";
        switch (opcion)
        {
            case 0:
                prueba = "select idUsuario,Nombre,ApellidoP ,ApellidoM ,Sexo from usuarios where Nombre ='"+dato+"';";
                break;
            case 1:
                prueba = "select idUsuario,Nombre,ApellidoP ,ApellidoM ,Sexo from usuarios where ApellidoP ='" + dato + "';";
                break;
        }
        SqlDataAdapter dataAdapter = new SqlDataAdapter(prueba, new BaseDeDatos().obtenerConexion());
        DataSet ds = new DataSet();
        dataAdapter.Fill(ds);
        DataTableCollection collection = ds.Tables;
        DataTable table = collection[0];

        foreach (DataRow row in table.Rows)
        {
            var data = new PruebaDeLLenadoDataGrid
            {
                idUsuario = row["idUsuario"].ToString(),
                Nombre = row["Nombre"].ToString(),
                ApellidoP = row["ApellidoP"].ToString(),
                ApellidoM = row["ApellidoM"].ToString(),
                Sexo = row["Sexo"].ToString()
            };
            dataGridUsuarios.Items.Add(data);
        }
    }